The present invention relates to a performance enhancing additive, and more particularly, to an additive for a basic vapor compression (VC) system such as cooling, air conditioning, heat pump and refrigerant systems, which dramatically improve the system's coefficient and performance (COP) and cooling capacity by lowering the thermodynamic load on the system compressor via lower pressure ratio and/or pressure difference, and which avoids the need for the more complicated approaches used in the past in enhance cooling capacity and performance.
FIG. 1 is a schematic diagram of a known VC or vapor-compression system. Commercial heat pumps and the like typically utilize other components in addition to these basic components shown in FIG. 1 such as oil separators, suction-line (liquid) accumulators, liquid receivers, mufflers, recuperative heat exchangers, reversing valves, high pressure and low pressure safety switches, thermal overload protection, and filter-driers.
A vapor compression heat pump utilizes a refrigerant evaporating at low pressure and low temperature to provide the cooling. This refrigerant vapor is then superheated slightly in the evaporator (to avoid sending incompressible liquid to the compressor) and compressed by the compressor to a higher pressure, thereby raising the condensation temperature so that the heat can be rejected to the environment as the refrigerant condenses to a liquid state. This liquid is then throttled in an expansion device to a two-phase mixture which enters the evaporator to complete the process. In many cases, the compressor which provides the compression of the refrigerant must be lubricated to maximize compressor life and a lubricant compatible with the refrigerant and the materials of construction must be utilized. The proper lubricant to use for a particular type of refrigerant, compressor configuration, and temperature range is well known in the art. For example, in a VC system using HFC-134a refrigerant, it has been proposed to used tetraglyme by itself as the lubricant, but tests have demonstrated that it is not an acceptable lubricant.
It is also well known that the lubricant used in the VC compressor becomes entrained in the refrigerant discharged from the compressor outlet and travels throughout the system. The refrigerant traveling throughout the system typically has an oil concentration of up to 5% by weight, the exact oil concentration being circulated being dependent on the compressor design, plumbing considerations, and the presence or absence of an oil separator, which is located directly downstream of the compressor outlet and returns the oil to the compressor.
Numerous attempts have been made in the past to improve the performance characteristics of the VC system. Each such attempt, however, introduces some additional complication or disadvantage into the system.
For example, FIG. 2 is a schematic diagram of a known heat pump system using a "vapor-compression cycle with solution circuit" (VCSC) to improve performance. A vapor-compression heat pump is a heat pump that uses the physical characteristic that the temperature at which evaporation (a cooling process) occurs and the temperature at which condensation (a heat rejection process) occurs are affected by pressure, so the working fluid (refrigerant) is compressed to alter the evaporation and condensation temperatures so as to pump heat. In this type of cycle, the traditional single-component refrigerant evaporator is replaced with a generator, where a liquid-phase two-component absorbent/refrigerant solution enters and the desorption of some or all or the generated refrigerant vapor from the two-component solution provides the cooling. In this system, the liquid and vapor are physically separated. This process absorbs the heat of vaporization of the refrigerant and the heat of dissolution. Various absorbent and refrigerant pairs have been proposed, with tetraglyme being one such absorbent in connection with various halocarbon refrigerants.
The VCSC generator is a combination of a heat exchanger and a liquid separator. The superheated vapor which is driven off and separated is compressed to higher pressure and the remaining strong liquid solution (a mixture of absorbent and refrigerant liquid) is pumped to a higher pressure. The separate liquid and vapor streams at the higher pressure are then combined in an absorber, and the absorption of the refrigerant vapor into the strong liquid solution rejects heat to the environment (heat of condensation+heat of solution).
The VSCS absorber is a heat exchanger for transferring heat to the environment and a device for exposing the liquid solution to the vapor. Typically the absorber is a falling film of liquid solution, which film passes through the vapor as it falls. The mixture accumulates at the bottom of this falling film where a heat-transfer coil (pipe) transfers the heat out of the solution and into a pumped coolant which flows through this cooling coil.
After leaving the VSCS absorber, the pressure of the resulting weak solution of absorbent and refrigerant is then dropped, via the throttling valve, and the mixture reenters the generator to complete the process. The vapor-compression cycle with solution circuit has improved COP because the latent heat has increased, thereby increasing the cooling capacity, for very little additional work. The work of a liquid pump is small, relative to the work of the compressor, because liquids are essentially incompressible.
The vapor-compression with solution circuit system is not practical, however, because it requires a balance between the liquid solution flow and compressed vapor flows which combine in the absorber. The flow rates of the vapor and the liquid solution change with cooling temperature and load, and the pump and compressor typically have different flow verses pressure characteristics. The end result is that under typical variable loads, the compressor and pump discharge pressures do not always match causing significantly reduced performance or even temporary failure of the system.
Complex pump, compressor, and/or by-pass control logic have not effectively resolved this very volatile control problem, and these control methods reduce performance and significantly increase complexity and cost. Another significant shortcoming of the VCSC cycle is that the absorber must expose the vapor to the liquid solution, via a falling film or some other mechanical way to allow for absorption of the vapor in the solution. This, in turn, requires a significant amount of space, thereby requiring large absorbers. Similarly, generators must provide sufficient free surface to allow for desorption of vapor out of the solution, and again this requires a significant amount of space. Generators, which are not quite as large as absorbers, are the second largest component in the system. Large size in such systems means heavier and more expensive systems.
Mainstream Engineering Corporation, the assignee of the present invention, has developed a substantially improved modification of the VC heat pump system with solution circuit shown in FIG. 2. This improved system which is shown in FIG. 3 is referred to as a chemical-mechanical heat pump (CMHP). U.S. patent application Ser. No. 08/347,095, filed Nov. 23, 1994, now U.S. Pat. No. 5,582,020, is incorporated herein by reference.
The CMHP has the basic benefits of the VSCS but the basic components of the simpler vapor-compressor systems. Instead of physically separating the refrigerant from the solution, however, the CMHP uses a special compressor to compress the two-phase (liquid-vapor) two-component solution together.
The low temperature generator heat exchanger in the CMHP does not also need to function as a separator, and the high pressure heat rejection absorber heat exchanger does not need to distribute the vapor to the solution because they always remain in contact. After leaving the high pressure/high temperature heat exchanger (and rejecting heat to the surroundings), the single-phase two-component liquid solution (absorbent plus refrigerant) enters the throttling valve (expansion valve). The pressure is dropped, and the solution enters the low pressure if heat exchanger where the heat transfer into the solution provides the cooling and drives some of the refrigerant out of solution and into the vapor phase. The superheated refrigerant and liquid solution mixture of the CMHP system is not separated, as it would be the VCSC, but instead is kept together and compressed in a two-phase (liquid-vapor) compressor to the higher pressure.
Typically compressors compress only a vapor and in fact the inlet vapor to a compressor is usually superheated to avoid any knocking caused by the attempt to compress a liquid. The CMHP compressor configuration is unique in that it must compress the liquid vapor mixture without knocking and no lubricant is used in the system.
The CMHP working fluid has greater latent heat capability when compared to a pure working fluid because a pure fluid (refrigerant) has only the latent heat of vaporization whereas this absorbent-refrigerant fluid also has the heat of vaporization and the heat of solution. The increase in latent heat increases the cooling capacity significantly, and the additional work to compress the liquid solution is less than the work required to compress the same amount of vapor. Efforts at Mainstream Engineering Corporation have demonstrated about a 7% to 10% boost in COP.sub.c for the optimum concentration of adsorbent mixed into the refrigerant charge.
FIG. 4 is a graph of CMHP performance data showing COP.sub.c verses average temperature lift for various solution concentrations from 0% absorbent (a typical vapor compression cycle) to 20% absorbent (percentage of absorbent plus refrigerant mass). The specific refrigerant was chlorodifluoromethane (HCFC-22), and the absorbent was N,N-dimethylformamide (DMF). Of course, numerous other possible absorbents and refrigerants can be used in the CMHP. The performance data demonstrates the benefit of using the liquid solution and also that there is an optimum concentration of liquid solution. For example, for the tests in FIG. 4, increasing the solution concentration from 0% (no solution) to 14% absorbent increases the COP.sub.c. Further increasing solution concentrations above 14% resulted in reduced performance.
Attempts have also been made to improve cooling performance via either additives or improved lubrication to reduce the friction loss in the compressor. For example, with regard to the former of the two approaches, U.S. Pat. No. 4,963,280 describes a composition for improving the energy efficiency of heat pumps by improving the heat transfer in the evaporator and condenser. A polar molecule which is a liquid halogenated alpha-olefin or liquid halogenated paraffin is postulated to form a Van der Waals bond with the metal surface of the heat exchanger thereby assumedly reducing the thermal boundary layer. This approach attempts to boost performance through a significant improvement in the evaporative or condensation heat transfer, but has not proved to provide substantial improvement in performance.
During our investigations of usable absorbents for the CMHP, we discovered that some of the absorbents are soluble in polyol ester (POE) lubricants normally used in HFC-134a vapor-compression (VC) systems and do not degrade the lubrication performance of the lubricant any more than the refrigerant itself degrades lubrication performance. One of ordinary skill would have assumed that the addition of absorbent into the VC system using a conventional compressor would have no beneficial effect on cooling capacity or performance, particularly because lubricant lowers heat transfer in the evaporator and condenser. Every effort is usually made to keep the lubricant in the compressor region. Because the lubricant is conventionally added to the VC system for the compressor itself and is not intended to serve a purpose anywhere else in the system, an effort is made through various complicated structural systems to keep the lubricant in the region of the compressor itself as much as possible.
The lubricant concentration in the refrigerant traveling throughout the VC system is typically in the 1-5% range. Because of this low amount, the concentration of additive that is present in the refrigerant/additive/lubricant mixture that travels throughout the system is significantly less than the concentration of absorbent normally used in the CMHP. Because of this reduced additive/refrigerant concentration one would expect the COP boost attributable to the additive/absorbent also to be significantly less. We have discovered, however, that certain concentrations of absorbent-type additives dramatically increase the COP boost and cooling capacity. COP is here defined as the usefully heating or cooling of the system divided by the energy required to obtain the heating or cooling. COP.sub.c is the coefficient of performance in cooling; COP.sub.h is the coefficient of performance in heating.
It is, therefore, an object of the present invention to provide improved performance for a vapor compression (VC) system with a very simple expedient.
It is yet another object of the present invention to achieve dramatic increases in cooling capacity and performance in a VC system without the need for a solution circuit or for any complex apparatus of the type needed for the known VCSC system or CMHP system.
We have achieved these objects with our discovery that the addition of certain weight concentrations of selected additives in the basic VC system provides substantial improvements in COP.sub.c and COP.sub.h.
An additive in accordance with the present invention has an atom which is an electron donor for hydrogen bonding with a refrigerant. Typical compound families with this feature include ethers, esters, ketones, aldehydes, anhydrides, amines, and amides. However, we have found that families with OH groups, such as alcohols, glycols, and carboxylic acids, tend to hydrogen bond with themselves rather than with the refrigerant and do not achieve the system performance benefits of the present invention.
Examples of additives for carrying the present invention include, but are not limited to the following:
tetraethylene glycol dimethyl ether (tetraglyme) PA1 triethylene glycol dimethyl ether PA1 diethylene glycol diethyl ether PA1 butyric anhydride PA1 tetramethylurea PA1 ethylene glycol diacetate PA1 N,N-dimethylformamide PA1 N,N-dimethylacetamide PA1 N,N-diethylformamide PA1 2-octanone
According to another aspect of the present invention, the refrigerant is a halogenated hydrocarbon having at least one hydrogen atom present in its chemical structure, such as HFC-134a or HCFC-22.
Another feature of the present invention is that the relative volatility between the additive and refrigerant should be significant, such that the additive can be considered non-volatile. We have found that this criteria can assessed via boiling point difference between the additive and the refrigerant.
Yet another feature of the present invention is that freezing point of the additive should be well below the lowest temperature in the cycle. Moreover, the additive should be of low toxicity and low flammability, and should also be environmentally acceptable.